My MonsterSpin is the new "foregrip" style A-5 barrel, I just took off the shroud and tape modded the Apex 2 tip on the end.
I have an old 98 flatline, and it has a tip on it, that I need to remove before I can put the Apex on it... But, it is on my workbench to do that as soon as I get some time... It is quite a bit longer than the A-5 flatline, but I would guess it will work fine.
The 98 flatline is threaded through, so you can't take it off as easily as the A-5 version.
I'm pretty sure the barrel itself isn't threaded on either version. I know for sure the A-5 has a little screw that locks it in place on the adapter that goes into the A-5, and I would guess the 98 is the same... I just can't remember...
The MonsterSpin is amazingly fun! By far the best barrel I have ever shot. But, if you get breaks in it, you pretty much have to stop and clean it before playing anymore, as the paint inside kills the accuracy. I hope to have some video this weekend that will show what happens when you run a broken ball through the system... I dropped a few pods, and didn't realize I had broken balls, until I ran them through my system coating it with paint!
Trails of Doom Stickers on Ebay!